What is the correct procedure when using a winged collection device (butterfly) to draw a light-blue top tube for a coagulation test as the first tube?

Correct Procedure for Drawing Light-Blue Top Tube with Winged Collection Device

When using a winged collection device (butterfly) to draw a light-blue top tube intended for coagulation testing as the first tube, you must draw and discard a waste light-blue top tube before collecting the tube that will be used for coagulation studies (option C).

Rationale for Using a Discard Tube with Butterfly Devices

When using a winged collection device (butterfly needle), the air in the tubing can lead to underfilling of the first tube collected, which is particularly problematic for coagulation testing:

• The International Society on Thrombosis and Haemostasis (ISTH) specifically recommends that "when you use a butterfly needle, the first tube of blood should be discarded" 1
• The air volume in butterfly tubing (typically 0.5-1.0 mL) can significantly affect the blood-to-anticoagulant ratio in the first tube collected
• Underfilling of light-blue top tubes leads to excess citrate relative to blood volume, which spuriously prolongs coagulation test results 2

Specific Requirements for Coagulation Testing

Coagulation tests are particularly sensitive to proper collection technique:

• Light-blue top tubes contain 3.2% sodium citrate and must maintain a precise blood-to-anticoagulant ratio (9:1)
• The American College of Cardiology advises that blue-top tubes must be filled to at least 90% of capacity to avoid falsely elevated results 2
• Underfilling can cause falsely prolonged PT/INR and aPTT results, with aPTT being more sensitive to underfilling than PT 2

Correct Procedure with Butterfly Needle

When collecting a light-blue top tube for coagulation testing using a butterfly device:

1. Perform venipuncture with the butterfly needle
2. Connect and fill a discard light-blue top tube first (this tube will be discarded)
3. After the discard tube is filled, connect the light-blue top tube that will be used for testing
4. Fill the testing tube completely to ensure proper blood-to-anticoagulant ratio
5. Gently invert the tube 3-4 times immediately after collection 2

Common Pitfalls to Avoid

• Do not use a red-top tube as the discard tube (option B) when specifically drawing for coagulation studies, as this doesn't address the proper blood-to-citrate ratio issue
• Do not skip the discard tube (option D), as this would lead to underfilling and potentially inaccurate coagulation results
• Do not assume butterfly devices cannot be used for coagulation studies (option A), as they can be used with proper technique 1

Evidence Quality Considerations

While some research studies have questioned the need for discard tubes in routine coagulation testing 3, 4, 5, these studies primarily focused on straight needle draws or healthy populations. More recent guidelines and studies involving patients on anticoagulation therapy 6 continue to recommend using a discard tube when collecting with butterfly devices, especially for specialized coagulation testing.

The ISTH guidelines specifically state that "independently of the needle type, the first tube of blood should be discarded" 1, providing the most authoritative guidance on this specific scenario.